Using interstitial cell concentrates and relatively pure Leydig cell fractions we plan to study the interaction of estrogens with DNA of Leydig cells and the mechanisms involved in the early spurt in DNA synthesis in strains of mice which have a high tumor incidence if treatment is continued. The type of DNA interaction will be compared with that in strains which do not have the early spurt and later have a very low incidence of tumors. By hybridization techniques and tracing the relation of the early DNA to that when malignant transformation has occurred we hope to determine any causal connection.